Two additional routines are supplied which don't require a psf identifier
but which might be of more general use -- they are psf_convolve and
psf_resample. The first convolves a psf with a 2-D kernel. The
second performs sub-pixel resamling of 2-D psfs by X and Y offsets. This
latter routine is particularly useful in applications which must allow
sources to appear at positions other than pixel centres.